fix(ci): downgrade artifact actions v4 -> v3 for Gitea compat

Gitea act_runner reports as GHES; @actions/artifact v2 (upload/
download-artifact@v4) uses the Twirp API Gitea does not implement,
failing with GHESNotSupportedError. v3 uses the artifact protocol
Gitea supports.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/build-nsInnoUnp.yml

@@ -47,13 +47,13 @@ jobs:
 
     steps:
       - name: Download Windows artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v3
         with:
           name: nsis-plugin-nsinnounp-windows-${{ github.ref_name }}
           path: artifacts/windows
 
       - name: Download Linux artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v3
         with:
           name: nsis-plugin-nsinnounp-linux-${{ github.ref_name }}
           path: artifacts/linux